LDOS 5.3.1 for the Model III - HITAPE/CMD Disassembled

HITAPE/CMD

Other Navigation

LDOS 5.3.1 HITAPE/CMD Disassembly - The 1500 Baud Cassette Interrupts (Model III)

HITAPE/CMD gives the 1500 baud cassette back to the ROM under LDOS. The ROM of the Model III reads a 1500 baud tape with the cassette's edge interrupts (bits 0 and 1 of the interrupt latch, port E0H), whose service routines are in the ROM (3365H and 3369H); LDOS 5.3.1 takes the RST 38H vector at 4012H for its own interrupt service (44A5H), which has no place for them. The manual (M-40-060) says so: the 1500 baud cassette loading was left out of the resident system for space, and HITAPE adds a routine to the DOS so that BASIC and CMDFILE can use 1500 baud tapes; without it a 1500 baud load does not load. The command is HITAPE; the parameter table also knows HITAPE (OFF), which the manual does not name.

The file is a load module of 456 bytes: a record of type 1FH with the text Copyright 1991 MISOSYS, Inc., All rights reserved, two load blocks for 5200H-5388H (393 bytes) and the transfer address record for 5200H. HITAPE copies a module of 28 bytes (536DH-5388H) to the top of the free memory, lowers HIGH$ below it, and points the RST 38H vector at it. The module sends the two cassette interrupts to the ROM and every other interrupt on to SYS0/SYS's service, entering it after its first three instructions with the stack as they would have left it. The manual adds that a SYSTEM (SYSGEN) done while HITAPE is in saves it with the configuration; the list of SYSGEN in SYS7/SYS holds both HIGH$ (4411H) and the vector at 4013H.

What HITAPE Does

About This Page

Load Blocks in File Order

The two load blocks of the file, after the copyright record: 393 bytes, 5200H-5388H. The transfer address is 5200H.

Sections of the Disassembly

Memory HITAPE/CMD Uses

The entry points of HITAPE/CMD, then the data of the file, the operands it changes, and the cells outside the file that it uses, by address.

Entry Points

Data in the File

Operands Changed While It Runs

Cells Outside the File That HITAPE/CMD Uses

5200H - Entry: the Banner and the Parameter

LDOS Ready loads HITAPE/CMD at 5200H-5388H and jumps to 5200H, the transfer address, with Register Pair HL at the rest of the command line after the word HITAPE. The command is HITAPE, or HITAPE (OFF) to take the module out again (the parameter table at 5364H; the manual, M-40-060, names only HITAPE). The Stack Pointer is kept in the operand of LD SP at 527EH, so every exit can leave from a clean stack; the banner is displayed and @PARAM reads the parameter.

5215H - HITAPE (OFF): Take the Module Out

With OFF the module $HT is looked for above HIGH$ (5296H). Not found: Module is not resident!. Found: when it is the newest module (it starts at HIGH$ + 1), HIGH$ goes back to the link word of its header, the HIGH$ before it was installed. Then the RST 38H vector at 4013H should get back the address the module saved at its +08H-+09H; but the code reads that word from Register Pair HL + 8 after the SBC at 522AH has changed Register Pair HL to the module less HIGH$ less 1: for the newest module from 0008H-0009H in the ROM (C3H 00H), so the vector becomes JP 00C3H and the next interrupt does not reach SYS0/SYS's service routine.

523CH - Install the Module

The module is looked for first: when $HT is already in memory, the RST 38H vector is simply pointed at it again. Else HITAPE must run at LDOS command level: @EXIT (402DH, or during a JCL job the copy SYS11/SYS keeps at 4065H) must not be taken over by a running program (a JP, C3H, there), and the command must not come from a program (bit 1 of 4758H); otherwise Can't -- Valid only at LDOS Command level!. Then the 28 byte module 536DH-5388H gets the HIGH$ (its link word) and the RST 38H vector (its +08H-+09H) of the moment and is copied to the top of the free memory, just below the old HIGH$; HIGH$ goes down by 28 and the vector at 4013H points at the module.

527BH - The End

Every exit comes through 527EH with Register Pair HL as the return code: 0000H for success, FFFFH after an error. The Stack Pointer is put back as LDOS left it (5200H); then success returns to LDOS, and an error does too unless a JCL job runs, which is ended at @ABORT.

528DH - The Error Exits and the Module Search

528DH gives error 2CH through @ERROR, which returns (bit 7 set); 52A6H displays Module is not resident! and 52AAH (inside the LD IX at 52A9H) Can't -- Valid only at LDOS Command level!, both through @LOGOT; all three end at 527EH with Register Pair HL = FFFFH. 5296H looks for the module through SYS10/SYS (request BCH) and returns its link word in Register Pair DE.

52B6H - The Texts and the Parameter Table

The banner, the two messages, the name of the module for the search, and the table @PARAM reads.

536DH - The Module: the Cassette Interrupts First (Copied below HIGH$)

536DH-5388H are 28 bytes copied to the top of the free memory (5271H) and entered from there through the RST 38H vector at 4012H for every maskable interrupt. The header: a JR over it, the link word (the HIGH$ before the module), 03H the length of the name, the name $HT, and the vector the module found at 4013H. The code reads the interrupt latch, port E0H (a bit is 0 while its interrupt is pending): bit 0, the cassette's rising edge, goes to the ROM's routine at 3365H and bit 1, the falling edge, to 3369H, the routines of the ROM's 1500 baud cassette read; any other interrupt goes on into SYS0/SYS's interrupt service at 44A9H with the stack as its own start (44A5H-44A8H) would have left it. The rows show the module at its address in the file; the JR and the jumps out of it work at any address.