From ed9f6538ee3469bee93547a066a33fced64718b8 Mon Sep 17 00:00:00 2001
From: Steve Kemp <steve@steve.org.uk>
Date: Tue, 16 Apr 2024 19:21:04 +0300
Subject: [PATCH] Implement function 12, SETDMA

This pull-request closes #25, by implementing the SETDMA syscall.
---
 cpm/cpm.go          |  9 +++++++++
 cpm/cpm_syscalls.go | 23 ++++++++++++++++-------
 2 files changed, 25 insertions(+), 7 deletions(-)

diff --git a/cpm/cpm.go b/cpm/cpm.go
index 8bc6bc2..82bec78 100644
--- a/cpm/cpm.go
+++ b/cpm/cpm.go
@@ -43,6 +43,10 @@ type CPMHandler struct {
 // CPM is the object that holds our emulator state
 type CPM struct {
 
+	// dma contains the offset of the DMA area which is used
+	// for block I/O.
+	dma uint16
+
 	// Syscalls contains the syscalls we know how to emulate.
 	Syscalls map[uint8]CPMHandler
 
@@ -121,6 +125,10 @@ func New(filename string, logger *slog.Logger) *CPM {
 		Desc:    "C_READSTRING",
 		Handler: SysCallReadString,
 	}
+	sys[11] = CPMHandler{
+		Desc:    "SETDMA",
+		Handler: SysCallSetDMA,
+	}
 	sys[13] = CPMHandler{
 		Desc:    "DRV_ALLRESET",
 		Handler: SysCallDriveAllReset,
@@ -184,6 +192,7 @@ func New(filename string, logger *slog.Logger) *CPM {
 		Logger:   logger,
 		Reader:   bufio.NewReader(os.Stdin),
 		Syscalls: sys,
+		dma:      0x0080,
 	}
 	return tmp
 }
diff --git a/cpm/cpm_syscalls.go b/cpm/cpm_syscalls.go
index 46d547f..65824ac 100644
--- a/cpm/cpm_syscalls.go
+++ b/cpm/cpm_syscalls.go
@@ -22,9 +22,6 @@ const blkSize = 128
 // maxRC is the maximum read count
 const maxRC = 128
 
-// dma holds the default DMA address
-const dma = 0x80
-
 // SysCallExit implements the Exit syscall
 func SysCallExit(cpm *CPM) error {
 	return ErrExit
@@ -112,6 +109,18 @@ func SysCallReadString(cpm *CPM) error {
 	return nil
 }
 
+// SysCallSetDMA updates the address of the DMA area, which is used for block I/O.
+func SysCallSetDMA(cpm *CPM) error {
+
+	// Get the address from BC
+	addr := cpm.CPU.States.BC.U16()
+
+	// Update the DMA value.
+	cpm.dma = addr
+
+	return nil
+}
+
 // SysCallDriveAllReset resets the drives
 func SysCallDriveAllReset(cpm *CPM) error {
 	if cpm.fileIsOpen {
@@ -290,7 +299,7 @@ func SysCallFindFirst(cpm *CPM) error {
 	// Create a new FCB and store it in the DMA entry
 	x := fcb.FromString(matches[0])
 	data := x.AsBytes()
-	cpm.Memory.PutRange(dma, data...)
+	cpm.Memory.PutRange(cpm.dma, data...)
 
 	// Return 0x00 to point to the first entry in the DMA area.
 	cpm.CPU.States.AF.Hi = 0x00
@@ -393,7 +402,7 @@ func SysCallRead(cpm *CPM) error {
 	}
 
 	// Copy the data to the DMA area
-	cpm.Memory.PutRange(dma, data[:]...)
+	cpm.Memory.PutRange(cpm.dma, data[:]...)
 
 	// Update the next read position
 	fcbPtr.IncreaseSequentialOffset()
@@ -429,7 +438,7 @@ func SysCallWrite(cpm *CPM) error {
 	offset := fcbPtr.GetSequentialOffset()
 
 	// Get the data range from the DMA area
-	data := cpm.Memory.GetRange(dma, 128)
+	data := cpm.Memory.GetRange(cpm.dma, 128)
 
 	// Move to the correct place
 	_, err := cpm.file.Seek(int64(offset), io.SeekStart)
@@ -544,7 +553,7 @@ func SysCallReadRand(cpm *CPM) error {
 			return 1
 		}
 
-		cpm.Memory.PutRange(dma, data[:]...)
+		cpm.Memory.PutRange(cpm.dma, data[:]...)
 		return 0
 	}